Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light. Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes and, in some cases, significantly higher efficiency for converting electric energy to light.
The cost and conversion efficiency of LEDs are important factors in determining the rate at which this new technology will replace conventional light sources and be utilized in high power applications. Many high power applications require multiple LEDs to achieve the needed power levels, since individual LEDs are limited to a few watts. In addition, LEDs generate light in relatively narrow spectral bands. Hence, in applications requiring a light source of a particular color, the light from a number of LEDs with spectral emission in different optical bands is combined. Thus, the cost of many light sources based on LEDs is many times the cost of the individual LEDs.
The conversion efficiency of individual LEDs is an important factor in addressing the cost of high power LED light sources. The conversion efficiency of an LED is defined to be the electrical power dissipated per unit of light that is emitted by the LED. Electrical power that is not converted to light in the LED is converted to heat that raises the temperature of the LED. Heat dissipation places a limit on the power level at which an LED operates. In addition, the LEDs must be mounted on structures that provide heat dissipation, which, in turn, further increases the cost of the light sources. Hence, if the conversion efficiency of an LED can be increased, the maximum amount of light that can be provided by a single LED can also be increased, and hence, the number of LEDs needed for a given light source can be reduced. In addition, the cost of operation of the LED is also inversely proportional to the conversion efficiency. Hence, there has been a great deal of work directed to improving the conversion efficiency of LEDs.
For the purposes of this discussion, an LED can be viewed as having three layers, the active layer sandwiched between two other layers. These layers are typically deposited on a substrate such as sapphire. It should be noted that each of these layers typically includes a number of sub-layers. The overall conversion efficiency of an LED depends on the efficiency with which electricity is converted to light in the active layer and on the efficiency with which light generated in the active layer escapes from the LED.
Improvements in materials have led to improvements in the efficiency of light generated in the active layer. However, a significant fraction of the light generated in the active layer is lost before the light can escape from the LED. Most of this light is lost through absorption in the various layers used to construct the LED. This mode of light loss is aggravated by the trapping of much of the light within the LED structure.
The three-layer LED structure is typically bounded on the bottom and top by materials that have a significantly lower index of refraction than the layers of the LED. As a result, a significant fraction of the light striking these boundaries is reflected back into the layered structure. One of the boundaries includes a transparent surface through which the light generated in the LED escapes. The other boundary is typically covered by a reflector that redirects light striking that boundary toward the transparent boundary. As noted above, the transparent boundary is typically covered by a material having a much lower index of refraction than that of the LED structure. Light striking this boundary at angles greater than the critical angle with respect to the normal at the boundary is reflected back into the LED structure. The critical angle depends on the difference in the index of refraction between the LED layers and that of the surrounding medium, which is typically air or a material such as a plastic. For LEDs constructed from GaN or similar materials, the difference is sufficient to result in a significant fraction of the light being reflected. This reflected light becomes trapped between the planar boundaries of the LED where it will be continually reflected until the light is lost due to absorption. In the case of conventional GaN-based LEDs on sapphire substrates approximately 70 percent of the light emitted by the active layer remains trapped within the LED.
Several techniques have been described to improve light extraction from LEDs, and hence, improve the light conversion efficiency of these devices. In one class of techniques, one of the outer surfaces of the LED at which the light suffers internal reflection is converted from a smooth planar surface to a rough surface. Each time trapped light encounters this roughened surface as it transits the LED, some of the trapped light will be redirected such that, at the next reflection from the surface, the light will strike the surface at angles that are smaller than the critical angle of the exit surface. Hence, a portion of the trapped light will now escape, and the process can continue, extracting more light with each round trip through the LED.
Prior art LEDs based on roughening a surface to improve light extraction typically employ a rough surface either at the top surface of the LED or adjacent to the substrate on which the LED layers are deposited. These approaches improve the fraction of the light generated in the active layer that escapes the LED once that light escapes the active layer itself. However, a significant fraction of the light that is generated within the active layer is trapped within the active layer due to internal reflections at the boundaries between the active layer and the cladding layers on each side of the active layer. These reflections are caused by a difference in index of refraction between the materials from which the active layer is constructed and the materials from which the cladding layers are constructed. For GaN based LEDs, the active layer is constructed from materials that have a significantly higher index of refraction than the cladding layers.